CIHM 
Microfiche 
Series 
(Monographs) 


ICIVIH 

Collection  de 
microfiches 
(monographies) 


Canadian  Institute  for  Historical  Microreproductions  /  Institut  canadien  de  microreproductions  historiques 


©2000 


Technical  and  Bibliographic  Notes  /  Notes  techniques  et  bibliographiques 


The  Institute  has  attempted  to  obtain  the  best  original 
copy  available  for  filming.  Features  of  this  copy  which 
may  be  bibliographically  unique,  which  may  alter  any  of 
the  images  in  the  reproduction,  or  which  may 
significantly  change  the  usual  method  of  filming  are 
checked  below. 

r~7|   Coloured  covers  / 
lj^   Couverture  de  couleur 

0   Covers  damaged  / 
Couverture  endommag^e 

□    Covers  restored  and/or  laminated  / 
Couverture  restaur^e  et/ou  pellicul^e 

I    Cover  title  missing  /  Le  titre  de  couverture  manque 

I I   Coloured  maps  /  Cartes  g6ographiques  en  couleur 

I      I    Coloured  ink  (i.e.  other  than  blue  or  black)  / 


D 
D 
D 


D 


Encre  de  couleur  (i.e.  autre  oue  bleue  ou  noire) 

Coloured  plates  and/or  illustrations  / 
Planches  et/ou  illustrations  en  couleur 


Bound  with  other  material  / 
Reli6  avec  d'autres  documents 

Only  edition  available  / 
Seule  Edition  disponible 

Tight  binding  may  cause  shadows  or  distortion  along 
interior  margin  /  La  reliure  serr6e  peut  causer  de 
I'ombre  ou  de  la  distorsion  le  long  de  la  marge 
int^rieure. 

Blank  leaves  added  during  restorations  may  appear 
within  the  text.  Whenever  possible,  these  have  been 
omitted  from  filming  /  II  se  peut  que  certaines  pages 
blanches  ajout6es  lors  d'une  restauration 
apparaissent  dans  le  texte,  mais,  lorsque  cela  6tait 
possible,  ces  pages  n'ont  pas  et^  film^es. 


L'Institut  a  microfilme  le  meilleur  exemplaire  qu'il  lui  a 
ete  possible  de  se  procurer  Les  details  de  eel  exem- 
plaire qui  sont  peut-etre  uniques  du  point  de  vue  bibli- 
ographique,  qui  peuvent  modifier  une  image  reproduite, 
ou  qui  peuvent  exiger  une  modification  dans  la  metho- 
de  noimale  de  filmage  sont  indiques  ci-dessous. 

Coloured  pages  /  Pages  de  couleur 


/      Pages  damaged  /  Pages  endommagees 


D 


Pages  restored  and/or  laminated  / 
Pages  restaurees  et/ou  pelliculees 

Pages  discoloured,  stained  or  foxed  / 
Pages  d6color6es,  tachetees  ou  p  .; 

Pages  detached  /  Pages  d6tach6ei 

Showthrough  /  Transparence 

j      I   Quality  of  print  vanes  / 


D 
D 


D 


Quality  in^gale  de  I'impression 

Includes  supplementary  material  / 
Comprend  du  materiel  suppl^mentaire 

Pages  wholly  or  partially  obscured  by  errata  slips, 
tissues,  etc.,  have  been  refilmed  to  ensure  the  best 
possible  image  /  Les  pages  totalement  ou 
partiellement  obscurcies  par  un  feuillet  d'errata,  une 
pelure,  etc.,  ont  M  film^es  a  nouveau  de  fa^on  a 
obtenir  la  meilleure  image  possible. 

Opposing  pages  with  varying  colouration  or 
discolourations  are  filmed  twice  to  ensure  the  best 
possible  image  /  Les  pages  s'opposant  ayant  des 
colorations  variables  ou  des  decolorations  sont 
film^es  deux  fois  afin  d'obtenir  la  meilleure  image 
possible. 


Additional  comments  / 
Commentaires  suppl6mentaires: 


Pagination  is  as  follows:  p.  195-217. 

La  pagination  est  comme  suit:  p.  195-217, 


Thic  item  It  filmed  at  the  reduction  ratio  checked  below  / 

Ce  document  est  fiimi  au  taux  de  reduction  indiqui  ci-dei*ou«. 


lOx 

14x 

18x 

22x 

26x 

30x 

/ 

12x 


16x 


20x 


24x 


28x 


32x 


The  copy  filmed  h«r«  hu  t—n  raproducad  thanks 
to  tha  ganarosity  of: 

Gersteln  Science  Information  Centre 
University  of  Toronto 

Tha  imagaa  appaaring  hara  mf  tha  bast  quality 
possibia  considarjng  tha  condition  and  lagibility 
of  tha  original  copy  and  in  kaaping  with  tha 
filming  contract  spacificationa. 


Original  capias  in  printad  papar  covars  ara  fiimad 
beginning  with  tha  front  covar  and  anding  on 
tha  last  paga  «vlth  a  printad  or  illustratad  impraa- 
sion,  or  tha  back  covar  whan  appropriata.  All 
othar  original  copiaa  ara  fiimad  beginning  on  tha 
first  paga  with  a  printad  or  illustratad  impraa- 
sion.  and  anding  on  tha  last  paga  with  a  printad 
or  illustratad  impression. 


Tha  last  recorded  frame  on  eech  microfiche 
shall  contain  tha  symbol  «^»  (maening  "CON- 
TINUED"), or  the  symbol  ▼  (meaning  "END"), 
whichever  applies. 


L'exemplaire  filmA  fut  reproduit  grica  A  la 
gAnArositA  da: 

Gerstein  Science  Information  Centre 
University  of  Toronto 

Les  imsges  suivantas  ont  tit  reproduites  svsc  le 
plus  grand  soin.  compta  tsnu  da  la  condition  at 
da  la  nattet*  de  t'axamplaira  film*,  at  an 
conformity  avec  les  conditions  du  contrst  da 
filmaga. 

Lee  exemplairaa  originaux  dont  It  couvartura  an 
papier  est  imprimis  sont  filmis  en  commanpant 
par  la  premier  plat  at  an  tarminant  soit  par  la 
darnitre  paga  qui  comporta  une  amprainta 
d'impression  ou  d'illustration,  soit  par  la  second 
plat,  salon  le  cas.  Tous  las  sutres  exemplsiras 
originaux  sont  filmte  an  commenpant  par  la 
pramiAre  page  qui  comporte  une  empreinta 
d'impression  ou  d'illustrstion  et  en  terminant  par 
la  darniAre  pege  qui  comporte  une  telle 
empreinte. 

Un  des  symboles  suivants  spparaitra  sur  la 
darniire  image  de  cheque  microfiche,  salon  la 
cas:  le  symbols  ^»-signifie  "A  SUIVRE".  la 
symbols  ▼  signifie  "FIN". 


Meps.  plates,  charts,  stc,  may  be  filmed  at 
different  reduction  ratios.  Those  too  large  to  be 
entirely  included  in  one  exposure  ere  filmed 
beginning  in  the  upper  left  hand  corner,  left  to 
right  and  top  to  bottom,  as  many  frames  as 
required.  The  following  diagrams  illustrate  the 
method: 


Les  cartes,  planches,  tabiaaux.  etc..  peuvent  Atre 
filmAs  A  des  taux  da  riduction  diffirents. 
Lorsque  le  document  est  trop  grand  pour  Atra 
reproduit  en  un  seul  clichA,  il  est  film*  A  partir 
da  Tangle  supArieur  gauche,  de  gauche  A  droite. 
et  de  haut  en  bas,  •n  prenant  *a  nombra 
d'imagas  nicessaire.  Las  diagrammas  suivants 
illustrent  la  mithoda. 


1 

2 

3 

1  2  3 

4  5  6 


MICtOCOPr   tISOUITION   TUT   CHART 

lANSI  and  ISO  TEST  CHART  No   2) 


^  ^^PPLIED  IMjGE_ln 

^^i  1653    fast    Mom    Street 

^"af  Rochester.    Htm    York  1 4609  '■\a 

'..a^  f''6)    *8?   -0300   ■   Phone 

^S  ''6)   288  -  5989      fa^ 


Keiirintcil  from    I  IIK  A-l  Ri'I'H  VMi  Al   Im  KNM,  Vil.  X\V.  Ni    ;,  April  1907 


"7 

CO 

1X1 


THE   CHARACTER    OF  THE  STAR   IMAGE   IN 
SPECTROGRAPH IC  WORK 


1.  S.  I'LASKEIT 


I'KIMKI)    M     rilK    t'NlVKRSirV    OF    CIllCAtiO    IKh-S 


'P 


i 


THK  CHARACTKK  OK    THK  STAR   IMACJK  IN  SPKC  TRO- 
(JKAIMIIC  WORK 

liv    J.    S.    I'l.ASKll  r 

The  object  of  this  paiur  is  to  descrihr  sonic  experiments  on  ilic 
size  and  form  of  the  star  ima^e  fiiven  hy  the  comliination  of  olijtctive 
and  correcting  lens,  with  an  investigation  into  tiie  i  iiisis  of  the 
observed  efficts  and  suggestions  for  the  improvement  of  existing 
conditions. 

The  equipment  of  the  Dominion  ( )i)Mrvatory,  Ottawa,  for  radial- 
velocity  work  consists  of  a  1 5-inch  telescope  with  a  Hrashear  visual 
ob";'ctive  and  photographic  correcting  kns,  and   a  spectroscope  of 
niversal  type,  also  by  Hrashear.     The  objictive  for  \isual  pur- 

•.s  is  excellent,  and  the  siKctroscope  is  admirably  adajitcd  for 
f^eueral  sj)ectroscopic  work,  but,  as  the  experience  of  others  as  well 
as  myself  has  shown,  is  not  suit,d)le  for  the  accurate  dt  termination 
of  radial  velocities.  Its  design  as  a  univers.d  siactroscoiie  dois  not 
give  sulTicient  stability,  and,  in  exinisures  of  any  length,  llexure  will 
not  only  ruin  the  defmition,  but  is  liable  to  introduce  systematic 
errors  in  the  velocities  obtained.  Pending  the  construction  of  a 
spectrograph  specially  designed  for  the  reipiired  purjuise,  an  attempt 
was  made  to  render  the  |)resent  instrumc  nl  capable  of  giving  acdir.ite 
velocity  values.  The  investigation  and  removal  of  the  known 
sources  of  error  led  to  thi'  discovery  of  the  aberrations  to  be  pn><  ntly 
described.  .\  brief  description  of  the  steps  leading  tlunto  i.  ,iy  be 
of  interest. 

Trusses  connecting  the  various  parts  of  the  instrunnnt,  where 
flexure  could  occur,  with  the  supporting  tul)es  were  applied  to  such 
effect  that  an  initial  displacement  of  the  spectral  !ini-s,  equi\alent 
to  a  velocity  of  30  km  per  second,  occasioned  by  a  movement  of  tele- 
scope and  spectroscope  through  two  hours  in  right  ascension,  was 
reduced  to  j\  km.  The  prisms  were  tirmly  damped  in  place,  without 
inducing  strains  in  the  glass,  by  screws  ]iassing  through  ''  v  base  of 
the  i)rism-box  and  the  minimum-deviation  linkwork  into  he  prism- 
cells.     The  slit-jaws,  originally  too  thick  on  the  edge,  were  reground, 


1(/) 


J.  S    I'LASKFTT 


;in(]  the  occulting  (ii^q)lir;ij;ms  for  star  ami  spark  lij;ht  \m  ri'  r>  moved 
from  tlu'  slit-hiail  ami  placed  on  m  imirptmltnt  franu'  attached  to 
the  sup|K)rtinK  tubes.  ' '"'  eomparison  apparatus  was  renKxleled, 
the  direction  of  the  spark  lieinf;  made  iransxcrse  to,  instead  of  parallel 
will),  the  slit  jaws,  and  many  otlur  smaller  di tails  were  (artfully 
attended  to. 

After  all  known  sources  of  error  in  the  spectroscoj'e  itself  had  lui  n 
ovireonie,  and  after  it  had  \mn  jilaced  in  thoroiiuli  adjusinnnt, 
it  wasfoimd  that  test  s])ectr.i  of  the  standanlvi  lot  ity  stars  occasion.dlv 
};.ive  Values  dilTeritif^  hy  as  much  as  ?  km  jur  si  cond  from  those 
obtained  by  other  ob^rMrs.  As  the  probable  i  rror  of  the  mean  of 
thi'  measured  lines  did  not  exceed  four-tc  nths  of  a  kilometer,  and  .,s 
all  the  other  known  causes  of  systematic  error  had  been  overcome, 
it  seemtd  probable  that  this  mij,'ht  be  I'iie  to  uns}nimetrical  distribu- 
tion of  tin  <tar  li,i,'ht  over  the  collimator  and  camera  lenses.  Kvidently 
such  unsymmetrical  distribution  can  cause  a  displacement  of  the 
lines  only  when  the  camera  is  not  in  exact  focus.  The  camer.i  was 
always  carefi'llv  focused  by  a  modit'ication  of  Xewall's  ntethod,  which 
readily  detei  displacemt  nts  of  the  sensili\e  surface  from  the  focal 
plane  of  l.ss  than  0.05  mm  in  a  foc.d  lenfith  of  375  mm.  Hut  as  the 
plates  are  supported  only  at  thi  ends  of  the  jihite-holderi,  dilTerences 
in  the  curvature  of  the  );lass  may  easily  cause  dilTerences  of  o.  i  mm 
or  more  in  the  position  of  the  center  of  the  sensitive  surface,  where 
all  measurements  are  made.  n  the  case  of  a  (lis[>Iacement  of  o.  i  mm 
from  the  focus,  a  distributioi.  of  the  star  li^;ht  on  the  collimator  objec- 
tive so  that  its  center  of  intensity  is  5  mm  to  one  side  of  the  axis,  is 
sulVicient  tocausea(lisi)lacem'  nt  of  the  spcctralline  j^t  X,'„  =7.1,,,  mm 
e(|uivalent  to  a  velocity  of  1 .8  km  i>er  second. 

.\n  examination  of  the  illumination  pattern  on  the  collimator  lens, 
both  visual  and  pliotoj^rajihic  showed  how  easily  such  or  even  greater 
displacements  of  the  center  of  intensity  could  occur  even  with  the 
utmost  care  in  guiding.  The  illumination  could  never  be  made 
uniform,  no  matter  how  the  relative  positions  of  slit  and  corrccting- 
lens  were  altered.  The  jiattern  was  either  a  diametrical  bar  parallel 
to  tile  slit  of  a  width  about  one-third  or  one-fourth  the  aperture,  or  else 
such  a  bar  w ith  the  addition  of  a  peripheral  ring;  while  a  very  slight 
movement  of  the  slit-jaws  to  one  side  or  other  was  sufTicient  to  cause 


.sy.U"  iMM.i:  IS  srixiKOiiKM'inc  wokk 


")7 


om-  side  only  of  tlu'  Inis  to  Ik-  ilhitnin.ilcl,  uiilmut  iMiisin^'  ,my 
a|'|>ri  I  i.iblf  cli.iii^;f  in  the  .ippiMr.inic  ol  tin  ini.ii,'i'  in  tlu  miiilint; 
lrli>"()|H',  >;iiidin^»  brin^  done  by  nifiins  of  li'^lil  minitii;  tiirou^li  tlu' 
>lil.  It  istM>\  to  SCI'  how  till'  ctntirof^inltn-ity  of  tlu  -t.ir  li^lil  miild 
bi'  di^plicKl  without  the  ol)Mrvi-r  biini;  ;iw,in  of  liu-  f.nl,  llui-'  i.iii-,- 
inj?  .1  (li>iil.utnu'nl  of  tlu'  st.ir  linrs  unlr>-.  tlu-  plate  utri-  in  ix.ict 
fotus. 

The  :ii(p<-:irann'  of  this  pattern  and  its  bihi\ior  for  ihini^'c  of 
>lil  |K)>ition  indiiMtiil  >phiriial  alurraliun  of  the  Kindeii-^ini;  >\>tein. 
I'hal  aberrations  of  sonu'  natiiri'  '.vere  present  \\a^  inili(  ati  il  luit  only 
bv  the  loni;  expo->ures  retpiired  ii]iw,ird  of  two  hours  for  i  star  of 
the  fourth  |ihoto'4raphie  niai;nitu<ii  but  aNo  bv  the  lari;e  elTeitive 
diameter  of  the  iniaj^e  as  >iu)wi'.  '.y  tlu-  wide  optiiiiii;,  o.j,  nun,  of 
the  slit  requireil  to  obtain  uniform  illumination. 

An  examination  of  the  correi  tint:  Kns  showed  that  part  of  ti\t' 
dilVuultv  might  arise  from  tlie  aciidinlal  in\ersion  of  the  divertjin^ 
elenuiit,  whieh  iiad  been  so  pl.ued  in  the  eell  that  surfaces  of  unlike 
eLir\aluri'  were  adjacent  to  each  otiit  r  ( )n  in-.trtini;  tiiis  (oncavc 
element  so  that  surfaits  of  like  radius  of  curvature  were  in  lont  ut, 
the  illumination  i>attern  became  more  uniform,  the  ri(|uirc  d  exjM)sure 
time  was  diminished  by  50  per  cent,  and  no  irmrs  of  a  jjreater  m  ij;ni- 
tude  than  should  be  expected  with  the  dispi  rsion  imployed,  apjicirtd 
in  velocity  determinations  of  standard  stars.  If  tlu'  diameter  of  the 
oi>ject-!:lass,  15  inc''es.  and  the  linear  dis]ler^ion  of  tlu-  spiitrograiih, 
18.6  tenth-meters  ler  miiiimeter  at  //*/,  l)e  taken  inio  account, 
the  ex|)o>ures  re(|uired  less  tii.m  an  hour  for  stars  of  the  fourth 
l>hotographii  magnitude  com j)a''  very  favorably  with  tliosi  of 
other  e<|uipnKnts. 

Notwitlistandin;;  tlie  great  imjirovement  shown,  iihotograjihic 
tests  of  tile  star  focus  for  dilTerent  tem])eratures  indicated  that  the 
star  >pectrum  was  much  wider  than  could  nasonably  be  accounted 
for  by  atmospheric  disiuri)ance,  and  I  was  led  to  make  thorough 
tests  of  the  character  and  diameter  of  the  image. 

To  determiiH'  whether  a  narrower  spectrum  lould  be  ol)tained 
bv  a  change  in  adjustment,  a  i)la!v  w.is  made  for  each  of  six  settings 
of  the  correcting-lens,  abovi'  and  below  its  computi'd  position,  over 
a  range  of  four  inches.     \  simple  device  applied  to  one  of  the  pl.ite- 


198 


/.  5.  PLASKETT 


holders  iniblftl  ten  succissivc  star  s|  tra  to  Ix-  made  side  hy  siilc 
on  iMcli  of  tlitsc  plalis,  at  dilTcrint  sitnnf^s  of  llu- slit  jxjsition  in  the 
ncij;hlx)rhood  of  th«:  star  focus;  the  sixiy  sjuitra  forniinj^  a  niord  of 
the  diameter  of  the  star  ini.iKe  under  x.iryin^  eoniiition>.  To  in>urc 
that  the  siKdrum  had  not  Ijeen  widened  by  a  drift  of  the  star  image 
aloni;  the  slit,  the  s|KCtrosto|ie  was  turned  in  jiosiiion  angle  until 
the  slit-jaws  weri'  parallel  to  an  hour  linle.  \\\  ojuning  the  slit 
o.  2  mm,  and  by  using  a  bright  star,  Vej^a,  a  fully  exiK)sed  linear  sjiee- 
trum  was  obtained  in  eight  or  ten  seconds,  evidently  with  no  chance  of 
widening  due  to  drift.  The  width  of  the  narrowest  part  of  the  narrow- 
est spectrum  on  eacii  plate,  presumably  where  the  star  was  in  focus  on 
the  slit,  was  miasured,  and  these  widths  ranged  from  0.085  '"  °  "5 
mm.  As  the  camera  and  collimator  objectivis  are  of  the  s;ime  focal 
length,  antl  .is  oni-  second  of  arc  in  the  focus  of  the  refractor  is  e(|uiv- 
ulent  to  0.OJ75  mm,  the  dianv.ter  of  the  star  image  according  to  this 
test  must  be  between  V'  iind  4.'5.  The  diameter  of  tlie  central  dif- 
fraction disk  as  given  Ijy  llie  formula  d        "  -—  is,  for  a    i^-indi 

objective  and  //>  light,  about  0^57,  while  the  actual  elTective  diam- 
eter as  obtained  from  the  width  of  star  spectra  is  live  to  eight  times 
as  great. 

This  enlargement  of  thi  d'lTraerion  image  may  be  due  to  three 
causes:  (1)  aberrations  in  the  sjH'ctrosco])e;  (2)  atmospheric  distur- 
bances; (3)  aberrations  in  the  system  of  objective  and  correcting-hns. 

I.  Abfrrations  in  the  spectroscope-  l\.  is  a  simph'  matter  to  deter- 
mine whether  the  wide  star  spectra  obtained  :ire  due  to  this  cause, 
for  by  direct  photography  of  the  star  image  no  aberrations  in  the 
s|)ectrosco[)e  can  atTect  the  result.  ;\  series  of  star  trails  was  there- 
fore made  on  ordinarv'  ]ilates  by  the  system  of  objective  and  correcting- 
lens.  A  small  plate,  held  in  guides  in  the  slit-cap  of  the  siKctroscope, 
could  lie  moved  in  these  guides  between  e.\|x)sures  so  as  to  make  a 
number  of  trails  on  each  pl.ite.  The  (ollimator  tube,  carrying  the 
l)late  with  it.  was  moved  by  the  rack  and  pinion  al)out  a  quarter  of  a 
millimeter  between  each  exposure,  to  insure  having  one  of  the  trails 
within  an  eighth  millimeter  of  the  focus.  .\  plate  each  was  made 
of  six  st;'rs  ringing  from  the  third  to  the  sixth  magnitude,  and  the 
width  of  the  narrowest  trail  on  each  plate,  corres|)onding  to  the 


S7  I  A'   nt  U.I    l\  .S77(   I  KiU.h:  I/' 


II  I  'AA 


I')') 


jKcilion  wluTr  'hi-  >t.ir  \\;i>  nm-t   :ii.irl\    in   Imti-.  u.i-  mr.,>iir.  .1 
.\lllii)ii^;li  till    iuti(liii(>n>  nl  sninu  Imtli  lor  ir.nl-     i^l  'imtr,!   vm  r. 
,il>i'M-  tlu'  .iMMHc  ,,'•       !   ;  in  .1  M  , lit  111  ;.  tin  ir.nl-  \m  ri  nci  .  nniinu 
oil-  l>ut  liroki  n  ,inil  ji^j^iil.  owinn  to  .ilnto-iiln  rii  ili-turh.inn -,  .itt'l 
llu    nuM-iin  iiunt-  wirr  in^nlc  in  two  \\,iv>:    I'lr-t,  of  I'u-  wiilili  oi 
n.irrow  -liori  p.iri-of  tin  ir    '  •  ulnri'  ilu  mi  ini;  h.nl  Im      iioiniiitariK 
stiMilv;  and,  -cionil.of  the  .,\(r,ij;<'  wiilili  ni  a  lon^^i  r  -trip  ol  tr.nl. 
In  tlif  rir>t  si-rii-of  niiM>-Mr(inrnt-  tin-  widiii^  \.irir(l  from  o  070  nini 
in  ihr  f  linti  r  -i.irs  too   110  mm  in  llu  lirii.;liti  r  »t,ir-.  wl.li  tin    at  r  i^t- 
wiilili^i  of  lonv;t  r  -iriif-  wirt-  .iIhuiI   .'o  \»r  tint,  '^rtii.i       >ii\«  1    llu 
witltli     of  -ix'dri  NMTf  |.r,ntii.iliy  tin    -,nu.  it   i.>  i\ii!.  m   ili.ii   \U> 
i.iiHt'  niu>l  l)f  >oiii;lit  in  llu-  >l.ir  im.iK>  il-tif,  .iml  i-  iu>i  ilm  to  iln  rr  1 
tion-  in  llu  ■']ii  i  iroM  opt-. 

.'.  Aliiiii^filicirii  ilisliirl'iiiiit^      Ntu.iU  in  hi-  p.i|it  tin    i|i-ii;n 

of  >|>ti  irojir.iph-'  li.i>  introilut  I  il  .1  m  ry  um  ;!  »i>iu  .on,  iliil  of 
inmor  (ii-k-,  .iml  In  -t.itt-  ili.it  almo-]  lu -■■  ■! -!iiri>,itu  t-  tnl.irm- 
tlu-  tl'liilivt-  liianuti-r  of  l!u-  ■~i,ir  !n,,ii;i  Si  '•  •  nl..p.,'t  nu  nl  ni.i\  Im- 
tliu- 1  iilur  to  boilih  ili-i>i.iit nil  III  :  In  im;i:,'t  .1  an  ii>  iiu ,in  )"'-iiion 
or  lii  tlu-  sprcailinj;  i>iil  of  1!"  1 1  riii.i.  ma^'t-  intt.  .1  nrirr  or  It  --  1  Ap.itul 
{■i\  ili-k.  lit-  iDii-itltr-  ih.ii  tlu-  .uiu.il  tlTt-tt,  -o  f.ir  ,i>  yt-ttiiv.^  li^hl 
tlirouuli  llu-  -lit  of  a  -pit  irour.ipli  i-  tonttrnnl.  i-  tlu-  -anu-  a-  if  llu- 
ima,i;t  tiin-i-li-(l  of  a  untral  tort-  from  1"  to  j"  in  ilianu  It  r  -iirronnilt  d 
l)v  a  mori-  t)r  li—  ililTu-t-  ami  ^'railu..ll\  illmini-liini^  porliiui.  llu-  wliol-- 
(lianitti-r  lu-ini;  in  llu  nt  iuliKorliootl  of  \"  or  :;".  If  \\t  .m  t  pi 
Ni-wall's  r-timalt-  a-  tDrn-tt.  .ind  if  \m-  rtnuiiilni  ill, it  in  no  i.i-i- 
\va-  a  -iiflu  it-nllv  lon^'  i-\po-iiri-  i;i\t-n  lo  allow  llu  millvinu  p.iri-  of 
llu-  irtiimi  ili-k  to  inirt-a-t-  llu-  witlili  of  -pi-tirum  or  trail,  llun  ilu- 
ili.inu  ii-r  of  llu-  imaiii-  i^ivi-n  l)\  llu-  'ii  "  1  liji-t  ,i\t  ,inil  lorrtiiini;- 
it-ns,  (.-vt-n  allowing  tlu-  i-\trt-nu-  limit  ,i--ii.;iutl  hv  Ntwall  tor  .iimo- 
phi-rit  (li-turhani  t-.  i-  nt-.irl\  i\\  it  t-  .1-  L;ri-.it  ,1-  it  -hoiiM  Im-. 

Il  i-  also  a  simplf  malttr  lo  u-i  ilii-  lotulu-ion  f\pt-rinit  nt.illy . 
.\-  llu-  ohji-ttivi-  i,'i\*'"'  i-N* till  lit  \i-ual  ill  linition,  it  m,iy  Im  sifily 
a— uniftl  llial  llu-  \i-ual  -1  r  im.,L;t-  i-of  norm.  1  ili.inu  it  r.  .\  mt.:-iiri 
nu-nt  of  tilt-  w  iillh  of  -pt-t  ir,i  ,m  1  trails  i)r(«iiit  1  il  1>\  ilu  \  i-ii.il  im.iL'r, 
anil  a  iomp:iri-on  with  tlu-  wiilth-  i;iv(-n  l)y  ohjniiM-  ami  mrrniiiiu 
It-ns  in  plioto.i;raphii  li.i^hl,  -houM  ,it  onti-  ilu  it  It  whitlu  r  llu  oh-t  r\til 

'  M.nilhly  .\,ili,,y  65,  So.s.    i.)c  5 


200 


J.  S.  PLASKETT 


clTcct  is  due  to  atmosi)heric  tri'mor.  The  correctinK-li-ns  was  thiTi-- 
forc  ri'iTiovid,  the  spictroscopc  was  adjustid  for  yillow  li^ht,  and 
spictra  wiTc  made  similarly  to  the  previous  ones,  lliou^ii  on  Cramer 
Isoehromalic  plates,  whicli  have  a  jjronounced  l)and  of  sensitiveness 
almost  identical  in  wave-kn^th  witli  the  turning-{)oint  of  the  color-curve 
of  the  objective.  The  widths  of  the  spectra  ])ro(luced  varied  between 
0.050  and  0.065  mm,  about  2",  Init  as  the  seein.i;  was  very  unsteady 
(about  i\  in  scale  of  5),  these  widths  are  doubtless  about  25  ])er 
cent,  f^reater  than  would  be  the  case  with  good  seeing.  Kor  the 
star  trails  the  same  make  of  plate  was  used,  light  of  shorttr  wave- 
length than  X.  5000  l)eing  al)S(irl)e(l  by  a  yellow  screen  of  ])lane  glass 
placed  in  contact  wilii  thi'  ])l.ite.  Owing  to  the  insensiliveness  of 
the  ])late  to  light  of  wave-lengllis  betwtcn  \  5000  and  \  5  100,  and  to 
longer  waves  than  X  5800,  only  the  light  wiiit  h  is  elTective  in  forming 
the  visual  image  c^'n  act  in  producing  the  trails.  As  befori',  the 
width  of  the  trails  varied  with  tlie  brightness  of  the  stars,  ranging  from 
0.025  mm  in  faint  trails  to  0.055  f""!  'i^  stronger  trails,  or  from  i" 
to  2",  while  the  average  width  over  a  longer  stri|)  of  trail  was  ai)out 
20  per  cent,  greatir.  .Xotwithstanding  the  bad  seeing,  both  trails 
and  sjjcctra  were  much  more  sharply  detmed  than  those  made  with 
the  correcting-lens  in  pliotographic  light  and  of  only  half  the  width. 

These  e.\[)erimenls  conclusively  prove  that  the  abnormal  width  of 
spectra  and  trails  in  photogra])hic  light  is  not  due  to  aberrations  in 
the  s])ectr()scope  nor  to  atmos])iieric  disturbances,  and  clearly  jjoint 
to  aberrations  in  the  condensing  system  as  the  cause  of  the  obsirved 
elTects.  .\  sliort  summary  of  the  experimental  data  will  render  this 
more  evident.  The  theoretical  diameter  of  the  central  disk,  or  ralhir 
of  the  first  dark  ring,  for  \  isual  light  \  5600,  is  o.'74,  for  pliotographic 
light,  \  4,^40,  is  o."57.  The  actual  width  of  visual  spectra  and  trails 
is  from  1"  to  2",  or  one  and  one-half  to  three  times  the  theoretical 
diameter.  Thi'  actual  width  of  photographic  spectra  and  trails  is 
from  }i"  to  4.' 5,  or  live  to  eigiit  times  the  theoretical  diameter. 

Some  further  information  regarding  the  si/e  and  character  of  the 
pliotogra])hic  image  may  be  gained  by  considering  its  elTectivc 
diameter  under  anotluT  aspect,  that  of  the  loss  of  liglit  at  the  slit. 
Referring  again  to  .^'ewall's  pa])er,  and  taking,  as  he  does  for  an 
example,  a  tremor-disk  of  5"  diameter  with  a  core  of  2",  we  find  that 


STAR  IMAGE  IX  SPECIROaKAPIIlC  WORK 


201 


a  slit  0.025  mm  wide  will  tnmsmit  ,^i  [ur  ctiit.  i)f  tlu-  inridirt  star 
li}^ht;  a  slit  0.0,^7  mm,  44  jht  ant.;  a  slit  0.05  mm,  58  pt  .nt.; 
and  so  on.  I  am  indiliud  to  a  suggestion  hy  I'rofts.sor  Campbell 
for  a  method  of  testinj^  this  theoretieal  result  experimentally.  .\ 
series  of  star  sjH'ctra  were  m.ide  at  dilTennt  slit -widths,  and  the 
resulting  inim-vities  were  comjiared.  .\s  it  is  practically  imiKissiblc 
to  make  ..  number  of  wide  spectra  of  uniform  intensity  thr(iu<^h()Ut 
their  width,  photometric  measurmients  c.innot  be  relied  ui)on  and 
recourse  must  Ik  "  ad  to  visual  estimates.  Sudi  estimatis  can  be 
made  more  accurately  if  the  exi)Osures  are  so  re};ulate(l  as  to  give 
spectra  of  e(jual  inttnsity,  an<l,  moreoxcr.  within  the  limits  of  ix- 
posure  lime  and  intensity  used  lure,  errors  due  to  the  ili.iracter- 
istics  of  the  plate  emjiloyed  are  to  a  >;reat  extent  avoided.  The 
spectrum  of  n  Lyrar.  the  star  usid,  is  ]iraclic.ii'y  continuous  exce]it 
for  the  If  series,  and  is  therefore  well  suited  for  the  estimation  of 
intensities,  while  its  brif^htness  is  such  tiiat  only  short  exix)sures  are 
re([uired.  Ten  ilitTerent  slit-widths  between  0.012  and  0.25  mm 
were  used,  and  ten  spectra,  one  throui;h  each  si  it -opening,  were 
made  ~ide  l)y  side  on  the  same  plate.  The  ex]K>sures  were  so  regulated 
as  to  render  the  resulting  spectra  as  nearly  ecpially  intense  as  possible, 
and  the  tmal  estimate  is  the  mear  from  a  number  of  ]ilales  and  from 
spectra  of  dilTerent  widths.  To  render  the  comi)arisons  more  direct, 
slit-widths  will  be  rejiresented  by  divisions,  a  single  division  corre- 
sponding to  0.025  mm,  and  ttie  relative  exjKisure  limes  will  be  reducerl 
to  a  unit  of  100  witli  a  slit-widtli  of  one  division,  0.025  ™'''^'  "•■  o'9t. 
the  normal  width  with  the  disjKrsion  eniployeil  here. 

The  following  table  slu)ws  that  the  exposure  miuin-d  is  inversely 
proiM)rtional  to  the  slit-width  until  this  reaches  0.1  mm.  Ir.iving  out 
of  accoimt  widths  less  than  a  single  division,  where  dilTracticmal 
loss  within  the  collimator  plays  an  imi^ortant  part.  It  also  shows 
that  with  normal  slit-width  less  ih.m  17  per  cent,  of  the  light  incident 
on  the  slit  is  transmitted.  In  Newall's  hy|K)thetical  rase  ,?i  per  cent, 
would  be  transmitted.  The  experimental  data  given  above,  using 
Newall's  metliod  of  calculation,  indicate  a  tremor  disk  H"  or  10"  in 
diameter  with  a  core  of  about  _v'5.  Jm'l.  as  the  previous  exixriments 
have  shown,  this  is  much  larger  than  can  l)e  accounted  for  by  atmos- 
jiheric  disturbances. 


302 


J.  S.  I'LASKF.TT 


Loss 


lAiiii:  I 

lit    I.Tc.iir  Ai  Si  tt 


CourAKAlUK  TlMK   FOR  I'jVM 

l\it  N>!rv 


I)i«. 

Mm 

Sfcs. 

IxiKrimcnlal 

("onipulcd; 

i 

O    OI  J 

0  45 

.qoo 

■  o.?? 

0  <)t 

100 

100 

li 

■°,(7 

1  .\> 

'>7 

70 

.050 

I    Sj 

50 

5  I 

■075 

2-7,? 

.V< 

'•.'< 

.  100 

3  "4 

28 

U 

5 

US 

4  55 

i"! 

,^i 

ft 

.150 

5  45 

21    7 

\i 

« 

.200 

/    ■  *  # 

<x  .>. 

.M 

lO 

.250 

0.07 

If.. 7 

,?• 

Tlu-  ;il)ovc  cxpirinunts  i>()int  (.oncliisivily  lo  abirmtions  in  the 
systim  of  objective  and  correctinj^-lens,  w  lit-n  used  with  i)hotogra])hic 
liglit,  ah  the  cause  of  llie  observed  elTects,  but  lliey  give  no  information 
concerning  the  nature  of  these  aberrations  beyond  indicating  in  a 
general  way,  from  the  appearance  of  out-of-focus  i)hotograi)hs  of 
spectra  and  trails,  that  spherical  aberration  is  ])resent.  It  was  decided 
therefore,  to  make  quantitative  tc-ts  to  ascertain  if  j)()ssible  the  nature 
and  magnitude  of  the  al;errations  and  the  best  means  of  removing 
them. 

The  most  simi)le  and  accurate  method  of  determining  the  zonal 
errors  and  axial  astigmatism  of  a  telesco])e  ol)jec!ive  is  Hartmann's 
melliod'  of  extra-focal  measurements.  The  ])rinciple  of  the  method 
and  tlie  measuremenls  and  reductions  mcessary  are  extremely  sim])le, 
while  it  gives  accurate  values  with  tlie  expenditure  of  com|)arativelv 
little  time  and  without  the  use  of  any  appliances  exce])t  such  as  can 
be  rea<lily  made  by  anyone.  For  the  benetu  of  those  who  have  not 
the  above  paper  at  haml,  and  in  order  to  render  the  ]iresent  article 
comiijete.  the  essential  principles  of  tile  method  will  be  brietly 
described. 

It  dej)ends  upon  the  <letermination  of  the  intersecting  point  of 
]Kncils  of  light  coming  from  ditTerent  parts  of  the  objective.  Sujipose 
a  diai)hragm  containing  two  small  openings,  etpiidistant  from  the 

■  /.litsihrijt  jiir  Inslrumenlcnkiiiulc.  24,  1.   ; ;,  07,  1904. 


.S7   lA'  IMMii:  I.\  .S7'/r/A'()(;  AM /•///(•  lli'AA' 


-'03 


ccntiT  ;intl  aloni;  ,1  (li.inntcr,  he  iilacid  over  tlu'  olijniiM'.  If  the 
dist.incc  httArcn  tlu'  ikik  il-;  of  li'^ht  lominL,'  from  tlusi-  oinniiiUM 
be  mi';isiir>'(l  at  two  jioint:-,  om-  witliin  and  oni-  without  tl\f  focus,  the 
])oiiU  of  interseetioti  of  llie  luneils,  and  lonseiiuently  the  foeus  lor 
the  particular  /one  in  (|Uebtion.  can  be  at  once  ol)tained  from  similar 


triani^les.  For  let  (/.,  Fi^.  i,  be  the  distanci'  between  llie  pencils  at 
the  scale  readinj;  .1,  within  tlu-  focus,  il,  the  <ii>tance  at  the  scde- 
readin^  .1^  bevond  the  focus.     Evidently  then  tiie  scale  rtailini;  for 

the  focus  .1  "'*•'>"'(,/'.'/)*•*'  ■''^-  ""'  '•'^'•'"'^''^  '^  ■'"'!  ''1 
mav  be  determined  directly  by  mii  ronieter  me.isuremt  nts  on  the 
pencils  from  a  star  or  distant  artiticial  point  source,  or  by  makiii'^ 
exposures  on  photoj;raphii'  platis  in  tlie  two  ])osilions  and  mea?urin,i; 
the  distances  between  tile  n-ultinj^  imai^es  by  a  ineasurini;  microscope. 
The  latter  method  is  j  ire  l<  Table  and  was  used  e\clusi\'l\ .  except 
that  tile  photoi^rajihic  determinations  were  checked  by  micrometer 
measures. 

A  /.one  plate  .1,  Fig.  2.  similar  to  that  <liscrilnil  by  Hartmann, 
was  employed.     Tiie  aiiertures,  exce|il   the   four  inner  ones,   were 


0 

% 

0 

J   c 

rj 

0 

(' 

00] 

J    ■' . 

.  C'  0 

0  0 

0 

c.^' 

c 

0 

0 

V 

\( 

'■^J 

0/ 

each  about  jq  mm  in  diameter,  and  the  mdii  of  the  nine  /oms  wi  re 
resjiectivelv  jS,  .17.  <>(>,  S;,  io|.  i_\:;,  i  |j,  .do,  and  17S  mm.  In 
order  to  determine  the  aslis^matism  alont;  liie  axis,  each  pair  of  open- 


304 


J.  S.  PLASKITT 


ings  is  (luplicatL'd  hy  a  second  similar  ])air  at  rii,'lit  anj^ks,  so  that  the 
foius  of  lach  zone  of  the  oljjcctivi'  is  (lilcTmimd  for  two  iKminls 
lKT|nndicular  to  <Mtli  otiu-r.  In  tlu'  case  of  tlii'  /.one  of  142  mm  radius 
the  focus  can  Ijc  obtained  for  four  elenunts  45°  apart.  Thus  ah 
cxix)sure  within  the  focus,  and  a  second  one  without  tin-  focus,  ^ive 
data  sulTicicnt  to  determine  tlu'  focus  of  each  of  nim-  /oms  of  the 
objective  in  two  directions  jHrpindicular  to  eath  other.  Tiuse  two 
directions  are  distinguished  from  one  another  in  the  measurement 
by  making  an  extra  aperture  in  tlie  zone  ])late.  whicii,  on  being  re- 
produced in  tlie  negatives,  ser\-es  t(  .dentify  tlie  origin  and  direction 
ol  the  angle  0. 

To  determine  the  zi)nal  errors  of  ulijeclive  and  correct ing-lens, 
the  zone  plate  was  placed  in  ])Ositi()n  in  front  of  the  objective  and  a 
small  photogra])hic  plate  was  ])lacc(l  in  the  guidis  in  the  slit-cap  of 
the  sjH  1  troscojie.  The  s]H'ctroscope  is  supi)orted  on  two  parallel 
tubes  carried  by  an  adai)ter  on  the  eye-end  of  tlu'  tih>i()pi',  and  can 
be  readily  moved  u|)  and  down  tlirough  a  range  of  ai)<)ut  20  cm. 
Experience  showed  tiiat  the  images  weri'  most  shar])ly  defmid,  and 
the  best  measurements  could  bi'  oljtained  when  the  jilalis  were  between 
6  and  10  cm  from  tlie  focus.  .\s  the  pliolographic  focus  was  to  be 
tested,  an  ordinary  Seed  27  plate  was  IJrst  tried;  but  it  was  not  found 
possible  to  make  very  accurate  settings,  as  the  ]iencils  from  the 
zone  ])late  were  spread  out  into  radial  spectra  owing  to  the  long  range 
of  wave  length  (^A.30CX3  to  the  limit  passed  by  t!ie  object-glass,  siy 
A.  3600)  to  which  such  a  plati-  is  sensitive.  Siveral  means  of  overcoming 
this  dillkulty  were  tried.  .\s  a  yellow  screen  in  front  of  an  ordinary 
I)late  did  not  imiirove  matters,  tlu'  dispersion  of  the  pencils  must 
evidently  be  ciiietly  due  to  tlie  light  around  II iS.  .\n  ordinary  lantern 
plate,  which  is  sinsitivi'  from  about  X  )()00  down,  was  therefore  next 
tried,  and  gave  good  images  capable  of  at  curate  measurement;  while 
if  a  yellow  screen  weri'  usid  with  suili  a  plate  tile  resultant  images 


were  again  ilongaled,  showing  tha 


]ir()l()ng((l  ix]iosure  entailed 


tlurebv  h;;d  extended  the  action  on  the  jilate  toward  the  rid  and 
nintroduied  the  lirst  ilifliculty.  .A  yiUow  or  red  star  was  used  in 
preference  to  a  white  or  blue,  as  limiting  the  action  in  the  \iolet, 
shortening  the  it'fective  range  (  lectrum,  and  thus  giving  images 
with  les.   spectral  dispersion  and  with  no  apparent  elongation. 


.s/.iA'  /.i/.i(.7-:  IS  srrxi Kihh.M'iiic  \\(^kk 


JO!; 


FoUfM  tsof  i\tr,i  foial  platis  win-  m.idr  whiih.oii  luini,' inc,i-'tiri<i, 
ndund,  and  avera^'id,  i;a\(   llir  hua!  iio>iti()n>  i)f  tin-  nitu-  /diks  as 
lahulattil  liilow   I'l'ahlc   11 1.      Ml   l'i>ur  nuasurcs  ar<'  in  substantial 
aKnt-niint,  whidi  dI  ioutm'  i>  ilo-t-r  for  llic  outrr  /oms  wluri'  tlu- 
(■()nv(rf,M;;v  of  ilir  juniii--  i-  i^rratcr.     'I'lu  re  thr  jn-olMMr  ;  rror  of 
a  >int;lf  (Ktcrminatioii  of  tin   focus  docs  not  ixiird  o.i  mm,  wiiiU' 
mar  till' nnlir  it  may  hi  a>  f^rrat  as  0.5  mm.     It  will  ln'  notiiid  tiial 
till'  forus  for  tin-  td^;i'  of  the  ol>jicti\r  and  torrii  linu'  U  ns  i>  u|i\vard 
of  2  mm  lon^'ir  than  the  fo(  us  mar  the  nnlir,  and  if  a>tii;matism 
hf  takin  into  aicoiint  also,  thr  ditTinncc  is  ),'riat<r  than  .'5  mm. 
'I'hi'  valurs  art'  plottrd  uraphiially  in  the  lurvr  (.1)  of  I'i.i,'.  .v  ihi- 
vtrtiial  distanco  hiin.i^  ma.unit'ud   some  -ix  or  s(\in  timis,  tlu-  a]>- 
])indi(l  stall-  ri|irt>inun)^  millimrtirs.      1  In  liori/.ont.d  line  is  drawn  in 
the  [Mj-itionof  foius  75.;^4that  j^ivo  the  >malU>l  i  inks  of  lontuMon, 
in  this  lasi-  0.04  mm  in  diamcttr.     The  astium.iti>m  will   iniriaM- 
this  to  sonu'  ixtint.  so  th.it  proliahly  tlu  di.inutcr  will  he  marly  2". 
L'nli>-  the  >lit  is  xt  ixaitly  at  this  niian  jiosiiion,  whiih  i>  not  likily, 
thi'  diannttr  of  the  ronfusion  disks  will  l>i- still  furtht  r  im  nax-d,  >o 
that  we  mav  ion>idi  r  .•"  as  a  modrrati-  istiinati.     It  mii>t  In-  n  r  iiin- 
hiTid.  how  I  \  IT.  that  in  sjnakini^  of  i  in  It-  of  ( (infu>ion  the  com  t  piions 
of  ^comi  triial  ojitiis  aloni  ari-  luin;^  lonsldind.  and  no  aiiount 
is  taken  of  dilTraition  ]ihinonniia.  whiili  may  ha\c  some  ilfnt  on 
the  m'omctriially  lali'ulatid  dimin>ion>  of  tin  Mar  di>k  n-^ultini;  from 
ahirralions  of  tin-  n.a.^nitudi'  htre  ]>ris<nt.     IIowt\ir,  the  ixjieri- 
ments  on  the  width  of  sjieitra  and  trails  showtd  ((.ntlu>iv'|y  that 
the  iiholo.tjraphii;  imaije  was  alunit  2"  j^reatcr  in  diameter  than  tile 
visual  im;  u'e,  ]iresuniat)ly  unalTeitid  hy  ..herrations.  and  this  aj^ries 
with  the  Ljeometriial  theory. 

To  determi'.ie  wliere  tile  aberrations  ari>e  it  is  necessary  to  aieu- 
ralelv  eompare  the  |ierformanee  of  the  ohjeilive  used  visually  w  ith  the 
])erforniame  of  the  olije(ti\i-  and  (oTediim  Uns  in  the  ]ihoto,i;r,i]ihic 
part  of  tile  siieetruni.  /ond  ti^ts  were  therefore  made  of  the  ohjer- 
tive  alone.  I'or  tin-  ])ur]M.-e  the  wa\e  lent;th  ol  the  lit^'ht  Used  niu^t 
he  limited  to  X-  5  )00  X  5S00,  tile  ran.Ue  to  which  the  eye  is  mo>t 
sensitive,  whltli  i^  the  most  luminous  in  tiie  >iieitrum,  and  wiiidi 
coincides  with  the  turning;  iH)ini  of  the  i<loriur\c  of  the  olijedive. 
Fortunately,  as  the  baiiil  of  color-sen>itivene>s  of   Cramer  Isociiro- 


206 


J    S.   Pl.ASKI.n 


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106    01 


niatic  |>l;it(.<  almost  ixailly  loimidis  with  ilu'  .^ami'  ri'j.;ion,  all  that 
is  iHTi'ssa ry  in  order  to  oljtain  iiliotoi^rapliic  test  plates  is  to  absorb  the 
blue  and  violet  li^'ht  by  a  suitable  screen,  ami  thus  eonfine  the  action 
to  the  visud  part  of  the  sjiectrum.  A  dee])  yellow  screen  with  plane 
parallel  surlaies  was  used  in  contact  with  the  plate.  Althou),'h  the 
])eprils  from  the  /.one  plate  are  displaced  slif,'h!ly  on  passing  through 
this  screen,  these  disi)lacements  are  projiortional,  and  the  onlv 
elTect  will  be  tolengtlun  the  focus  for  all  the  /.ones  by  the  same  amount, 
about  one-third  the  thi(  kness  of  the  screen,  witliout  in  the  least  alter- 
ing the  relative  ]M)>itions  of  the  pencils.  .\n  e.\|)osure  of  about  a 
minute  on  ('apclln.  ilirough  tlie  screen,  with  thi  plate  from  Oo  to  loc 
mm  from  tiie  focus,  gives  a  negative  of  good  intensity  in  whidi  tlie 
images  of  the  pencils  are  ipiite  round  and  free  from  any  noticeable 
spectral  elongation,   thus  allowing  accurate  measurement. 

Five  sets  of  e.xirafocal  e.\|K)sures  were  made  in  the  visual  i)art  of 
the  si>ectrum.  and  tiu-  n.ean  values  resulting  from  the  nieasureniint 
and  reduction  of  these  plates  are  given  in  Table  II  and  plotted  graphi- 


5/MA'   /U.K./;  /.Y  SI'ECl  H(H:K.\rillC  \\(^KK 


In..   ;.    -AitKil  OilTiTcnci  s  cf    Incus 

callv  in  t  un  t'  /•'  of  Kig.  ,^     An  examination  of  tliis  curvf  shows  that 
no  point  or  focus  is  at  a  j^nati-r  (iistance  than  o.  2  mm  from  thi'  iK)sition 


20S 


J.  S.   ri.ASKr.TT 


of  ini'Mn  f()(  us,  shown  by  the  hori/ont.il  line,  i\(r])i  ;i  smiill  rii;ion 
nt.ir  tin-  ( cnicr  of  ilu'  ohjictivc  wliirli  lias  a  l()ni,'ir  fonis.  Thf  ilTci  t 
of  this  n  1,'ion  on  llic  ]>(  rforni  ncc  of  the  ol)jiiti\r  nuisl,  Iiowimt,  \w 
i\cnilint,'ls-  sm  ill,  owini^  to  its  small  ari'a,  K>s  than  om-tinth  of  tlu' 
()l)ji(ti\f,  and  to  till-  uiak  ( oiUAT-^cnry  of  llu'  pmiils  ]pro(i'i(lint^ 
from  it.  In  f.iit  if  H.iriminn's  iriirrion  7'  as  to  the  (|iiality  of  an 
()l>jiili\i'  1)1-  ii)ni|)iiti<l  from  tlu-  al)ovf  nuan  valius,  it  is  found  to  hf 
0.141.  Aiiordiiif^  to  this  d  issitkation  an  ohjictivc  is  nio<liratilv 
("ma— i.i,'"i  i^iwxl  whin  V  is  ^'ri;!tir  than  i  .5,  ;;<hx1  whin  7'  is  Ixtwiin 
o. ;  ,ind  1.5,  .ind  I'xcirdinj^ly  ("htrvorraf^cnd")  <.;oo(l  when  7'  is  less 
th  m  0.5.     In  till'  idi'.d,  ahsohitily  /.on-kss  ohjriti\r  7'  is  o. 

I'".viilinlly  till'  ohjcitivf  whrn  used  visually  is  of  thi'  viry  first 
(|U  dity,  and  the  alHTr.ilions  a]i|>iar  only  whin  it  is  usid  in  lonjunition 
with  an  auxiliary  lornitor  for  s|>iitroj,'ra|>hic  work.  Whithir  the 
.ihrrriiions  tluri'  ])risi'nt  arc  dui-  to  the  lornilinj^  lins,  or  to  the 
ohjrilivi'  whin  usid  in  the  iiii()to,!.,".,|ihic  i>art  of  tlii'  s|if(  truni,  remains 
to  lie  ditirmined.  For  thi>  jpurpose  a  further  aipjiliealion  of  Mart- 
niann's  method  was  necessary  to  Imd  the  color  curves  of  the  ohjeclive 
ah)ne,  and  of  the  system  of  ohjeclive  a.nd  correi  tinj;-lens  for  a  number 
of  /ones.  It  was  iiopid  that  such  ohserv.itions  would  throw  liijht  on 
the  cause  of  the  al)errations  and  suj,'f,'esl  a  [tossihle  remedy.  They 
would  also  serve  as  a  che(  k  upon  tile  /.one-pl:ite  determinations,  as, 
in  this  case,  no  spectral  iiisiHr>ion  .if  the  pencils  could  alTect  the  accu- 
racy of  setting;.  'I'o  Imd  such  color-curves,  the  pencils  of  lifjht  coming 
from  a  /one  jilale  fall  on  llie  spectroscope  slit,  and  the  distance  between 
the  resulting;  spectra  taken  with  the  slit  within  and  beyond  the  focus 
^ives  a  measure,  cilcul  ited  in  llie  same  way  as  before,  of  the  focal 
position  of  any  desired  wave  length  for  any  |).irticular  zone. 

It  w.is  decided  to  determine  the  color  curves  of  eijjht  zones  of 
,^8,  57.  76,  05,  114,  I},!,,  i-,2,  171  mm  radius;  and,  to  ])revent  the 
spectr.i  from  merf^ini;  into  one  anotlur,  two  /one  jilates  were  rec|uired, 
one  (H),  Fii^.  2,  ol  the  four  /ones  of  57,  05,  i  ;;,^.  ami  171  mm  radius, 
and  the  other  (C),  Fig.  2,  of  the  remaininj^  four.  The  central  open- 
inj^s  were  each  20  mm  sijuare,  and  the  outer  20  by  25  nmi.  The 
/one  plates  were  so  placed  on  tlie  objective  that  the  row  of  openings 
w.is  parallel  to  an  hour  circle,  and  the  spectroscope  was  turned  in 

*  /eitsfhri/t  jttr  Instrumcntnikitndt',  24,  40,  1904. 


STAR  l.\t.U;i:  l.\  SPECTRtHiKM'im     \\i>KK 


.•0() 


|X)situm  iin^li'  until  tlu'  -lit  was  i)ar,ilitl  to  tin  opmini;^.  in  onhr 
that  imj^uLiritii's  in  driving  wouM  not  w  idi  n  tin  >i»itra.  I  u  iliniin 
ish  the  t'X|><)surcs  as  mm  li  as  jKissihli-,  lirii^ht  siar>,  l'<,i,''/  .mil  Sinus, 
wiTf  usrd  and  thr  slit  was  widily  o(Hni.d.  ,  s  no  in.  uur.,i>  would  In- 
thiTi'bv  intRMlnct-d  in  thf  dist.incc  liilwun  tin  ^]>ntr.i.  Tlu  >  xpo 
sun-s  win-  madi-  on  a  nii^hl  wntn  ihi'  trmpt  ratun-  w..s  m-.  ri\  -iiiion 
ary.and  wrrt'.irr.in^id  in  thr  following  order: 

I'latr  i;  /"iir  I'l.ili-  iH)  \in.  J.    >lit  ,il«.ut  50111111  williiii  llu    l"i  ii^ 

.;  iD  "        ■'  ;o  ' " 

5;  (C')  "        "  40  "     Ih\.'Ii1 

4;  («)  "        •'  40 

This  imjcedure  was  followed  to  avoid  a>  f.ir  is  iio".-il>li'  any  n  la  • 
tivc  (Usi)l;icemenl  of  the  foc.il  deiermin.itions  of  the  two  m  1 ,,  due  lo 
slight  changes  of  temjieniture  of  the  ohjeetive.  That  no  nu  iMir.ihle 
displacement  has  occurred  is  shown  by  the  continuity  of  tlie  /.on.il 
curves  of  Fis^.  ,^  dniwn  from  the  comhin.ition  of  the  two  ^epar,lte 
determinations,  and  by  their  af;reenunl  with  tlioM-  m.ide  by  tiie 
re^^ular  /.one-plate  method. 

Each  of  these  plates  cont.iins  ei;;ht  sjiectr.i  sidi  by  side,  one  from 
each  light  i)encil  transmitted  by  the  /.one  )>l.ite.  .md  the  po-ition  of 
the  focus  for  each  zone  and  for  any  desired  wave  lenu'th  in  the  ran^,'e 
on  the  plate  can  be  determined  in  ex.ictly  the  same  w.iy  ,is  before. 
The  hvdrogen  lines,  in  the  I'lr.st  type  stars  usdi,  >erve  as  d.itum  marks 
for  the  identil'ication  of  wave  leni^llis,  and  me.i>urement>  w(  re  m.ule  .it 
eleven  [wsitions  between  X.  .^gjo  and  X.  SO.iO-  'll'*'  corresjxindin^' 
focal  iwints,  as  calcul.ited  from  these  me.isurements,  are  given  in 
Tat  .  Ill  for  eight  zones  of  the  objective  alone,  and  in  T.ible  IV 
for  the  same  eight  zones  of  the  objective  with  corni  ting  lens,  the 
hitter  being  about  40  mm  nearer  the  focus  than  its  computed  [H):,ition. 

Tile  reason  for  using  the  correcting  lens  below  its  computed  position 
at  (mce  appears  on  inspection  of  Fig.  4,  which  represents,  in  their 
correct  relative  jjositions,  the  color-curves  of  a  medi.m  /.one  of  108  nmi 
radius,  determined  in  exactly  tiie  same  way  as  above.  Curve  .1 
(Fig.  4)  is  the  color-curve  of  the  visual  objective  between  the  limits 
\  6250  and  X  3()70,  which  shows  that  the  minimum  focus  i^  at  about 
\  5(100,  exactly  in  its  computed  ])osition.  Curvi'  B  is  the  color  curve 
of  the  system  of  objective  and  correcting-lens  between  \  6250  and 


3IO 


J.  s.  PLASKirr 


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Fli"..  4. — Ciilor-Curves  for  a  Modian  Zone 

A.  3970,  wliich  shows  that  llic  minimum  focus  is  at  about  //^,  instead 
of  //7,  its  computed  jMJsition.     When  the  correcting-lens  is  moved 


STAR  IM  U.F.  I\  SricrfH^CKM'IIIC  UCAA. 


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down,  a\v;iv  from  tlu-  ()l)jirlivi-,  some  40  mm  wi'  ^ti  (  urvc  C,  and  .it 
70  mm.  (urvf  /).  In  curvt-  C  the  minimum  f(H  u-  is  marly  at  //-/, 
and  in  D  at  X  44(10.  Kvidi'ntly  tlif  lowirin;,'  of  tlu'  corrtrlin^  li  ns 
somt-  40  mm  rlTi(  tsconsidiTahlr  iniiirovinunt  in  tin-  color  torrid  ion 
without,  as  thi'  earlier  experiments  showed,  apiireciahly  enbrtjinj; 
the  ima^i',  and  the  lens  has  been  used  in  ihis  position  almost  from  the 
tirst. 

Althouf^h  all  the  data  in  rej^ard  to  the  eomjilete  coloi  rves  :;re 
given  in  ''"ables  111  and  IV,  still  the  aitu.d  curves  dr.iwn  from  these 
figures  show  all  the  cimditions  at  a  glance,  and  are  hence  v,  ,.Mi 
giving.  To  jireAent  too  great  a  confusion  of  lines,  tlie  curves  for 
four  zones  only  (zone  jilatc  (B),  Fig.  i),  of  57,  95,  i,?.^  171  mm 
radius,  arc  shown  here  in  Fig.  5,  the  upper  curves  being  of  objective 


312 


J.  S.  PIASKEIT 


1  "1 r- 


■  T"        ■  ■       f  - 


I'li;.  5. — t'liIorCurvis  uf  luur  '/.mws  of  Objedive  and  uf  Objective  with  (orrt-ctor 


alont',  the  lower  of  objective  and  correclor.     These  cunxs  show  at  a 
glance  that,  in  the  photograjjhic  part  of  the  spectrum,  the  focus  for 


.STIR  /»/.!(,/  /.v  .srtci K(h,K-  \riii(-  ln>^'^• 


-''.^ 


thr  rili^f  i>(  tlu'  oltjcdivi'  i-  lon^jiT  ili.m  tin  loni-  fur  ilun  iiii  r.  ili.il 
it  his  ni'i^itivf  sphiTitMl  iilurritinti.  I  lii-  i  limtn.itii  (liiiVrnm  of 
s]ihiTiiMl  al>iTr  ition  ii  intunut  in  two  |i  .rt  ohji  i  ii\i'^  of  tlii  onlin.iry 
Hl;issi's,  ;inil  till'  i»nlv  niTicflv  i>  t<i  i(imi«n^il<  lor  it  liv  iniriHlurinR 
the  lorrcd  amount  of  |K)»itivc  alxrr.ilion  liy  tin  i  orn  i  tin;;  It  ns. 
Howrvcr.  till-  lowtT  (iirvrs  ,ho\v  th.it.  in^tcaii  of  tom|M  n^itini;  for 
this  ihroniilir  diiTinnif,  thf  rorn'dini;  h  ns  h.i>.  on  tin  inntrar\. 
incrcasid  it  sonuwtiat,  and  the  foi  u>  for  ni.irv;in..l  ri\  •  i-  iipu  n!  of 
2  mm  lonpr  than  thf  focus  for  untnl  ri\-..  I  hi>  ai;rn-  ahno^t 
fxuctly  with  thi'  previous  (Ictirniinition  of  tJK  /oiial  loi  i  of  oliju 
live  ;in(l  corrector,  and  is  jjcmmI  evident i-  i>l  thi  ^uli^taiuiil  atiuraty 
of  the  determinations.  Heft)re  leavini;  thf^e  curt  t>  it  ir.i\  Ik  iMiinietl 
out  tha'  the  crossing; of  the  curve  from  tin  •i~  mm  /one  ovt  r  tiie  oiin  rs 
in  pissing  from  sliort  to  lont;  waves  is  ihii  to  tiie  loni;t  r  t  ii~  ol  the 
central  /.ones  in  the  visu  d  part  and  is  furih.r  evidtU'i  ai  t,i\or  of 
the  aicuruv  of  ihe  thtermin.ition^. 

To  ohtain  a  s'iii  more  strikin'^  ti>mp,iri>on  of  l!if  i.uit-  .md 
mafjnitude  of  the  aberrations  present  in  tin  ■•\>uni,  ilu  color  <  ur\<s 
can  he  ])resentvtl  in  another  form,  th.it  of  /on.il  foi  i  turws  like  .1 
and  /'",  I'"i^.  .^,  previously  iKternuned.  We  have  the  i  olur  cur\e>. 
or  the  jiosifons  of  fot  us.  of  tlie  whole  jihoiourapliii  rt  i;ion  for  ei^ht 
zones  of  the  ohjectivi  in  Tahiti  HI  and  I\'.  and  thi>e  tan  In  rt,  dily 
plotted  in  the  same  way  and  on  the  s.ime  scale  as  .1  ,  ml  7'.  Ki^;.  ^ 
If  such  curves  were  plotlid  for  every  w,i\i  lePfZtll  in  t!it>e  tahles, 
thev  would  show  a  striking  aj.;reenient  in  form,  hut  1  h:i\e  ^atislud 
mysilf  with  representing;  ih'-  positions  <..  lu  lotus  of  t  ii;lit  /ones  lor 
Uy,  the  wave-len,i,'th  for  wiiich  the  system  w.o  i ominiltil,  .lud  lor  the 
mean  of  X^^^o,  4,^40,  4440.  and  4,^50,  the  ran^e  of  s;iittrum  umiI 
here  in  velocity  di  terminations.  /•;,  Fij,'.  .^  is  the  t  urve  ft  lly  u(  the 
ohjective  alone;  C  is  the  curve  tt>r  Jlyni  objective  anil  torrtilor. 
/)  is  the  curve  for  \  4250  to  \  4550  of  the  objettive  :  loni  ;  H  i>  the 
curve  fur  X.  4250  to  X  4550  of  the  objettive  and  t orret  tor. 

A  ctnni)arison  of  curves  /)and  I:  with  /•'  shows  in  a  -trikini;  manner 
the  chromatic  dilTerences  of  si>herical  aberr.aion  in  the  objittive 
when  used  with  photoi,'raphic  liLiht.  If  we  leavt  out  of  at  count  or 
allow  for  the  deviations  in  the  central  /.oms,  we  see  that  the  ftxus 
of  the  outer  is  about  1.8  mm  lonj;er  than  the  focus  for  the  central 


214 


J.  s.  I'LASKJ/rr 


/.()nr>,  ;i  li,mirc  tiiat  af^rtts almost  cxadly  w illi  the  comjuitrd  (lilfcnncc 
as  fiirmVlnd  nu'  In  I'rofissor  Hastinf,'s.  A  ( omiiiri-on  of  dirvts 
.1,  li.  and  ('  willi  P  and  /•,'  shows  that  this  dirunna',  instiad  of  Ininp 
rimovfd  or  diminislud  l)y  llu-  inlnnliK  tion  of  llic  corrn  ting  kni- 
lias  on  till-  lonirary  Incn  incrtasid  by  alxmt  0.6  mm,  sf  ;hat  \h 
(liffircnii  in  fom>  hilwiin  ouUr  and  (intral  /.ones  i.-  now  alioiu 
2.5  mm,  wliidi,  a>  hifori'  slatid,  will  gi\i-  a  confusion  disk  marly  2" 
in  dianutir.  1  wish  lo  jioint  out,  htforc  li;,\in,u  thisr  iur\i>.  how 
tiu'  form  of  the  curve  is  maintained  throiifihout  from  /•'  u\>  lo  .1 
e.\ce])t  that  the  axis  of  the  curve  is  inclined  downward  hy  tlii'  chro- 
matic dilTerenci'S  in  the  photoj^raphic  region,  and  further  tilted  hy 
the  introduction  of  the  torrecting  lens.  To  show  this  I  have  dotted 
in  ap|iro\imati'  ]iositions  of  su(  h  a.xes  in  tlu-  lurves  /•,'  to  .1  to  corre- 
s])ond  witii  thi'  horizontal  axis  in/''.  It  will  hi'  notind  that  the  irregu- 
larities in  til'  visual  curve  an-  continued  throughout,  hut  in  an 
int'  n~it'ie(l  form,  as  is  to  \n-  expectid  wlun  it  is  considend  that  the 
ol)jetti\e  was  i()mi)Uted  and  figurid  for  visual  work,  and  its  use  in 
the  ])h()tograi)hic  region  wilii  an  auxiliary  corrector  was  only  a  second- 
ary consideration. 

I  see  no  reason  to  doubt,  howiver,  if  sullicit'nt  positive  abi  rr,  lion 
were  left  in  tlu'  (oiieding  lins  to  com])er,.-ate  for  the  nig:. live  aber- 
ration introduced  by  the  chromatic  dilTennces,  that  the  jurformance 
of  the  system  could  be  much  improveil,  although  it  is  not  likily,  from 
the  magnifying  of  the  unavoidable  zonal  aberrations,  th;  t  it  would 
equal  its  visual  quality.  If  the  curve  .1,  Fig.  ,^  representing  the 
present  condition  of  the  s\stem,  could  be  tilted  through  the  angle 
between  the  horizontal  and  dotted  lines,  by  such  a  change  in  the 
correcting-lens,  the  resulting  confusion  disk  would  certainly  have 
a  diameter  less  than  half  its  present  magnitude,  while  the  ])ercentage 
of  the  ill'  idem  star  light  transmilled  by  the  slit  wouiil  be  consider.. bly 
increased,  jirobably  doubled,  with  a  pro])orlionate  diminution  of  the 
rei|uire(l  exposuri'  times  for  stellar  spictra. 

Such  an  imprtnenient  would  be  well  worth  considerable  etiort, 
anil  I  ha\i  '  vn  in  communication  with  the  Urashear  C'om]i.!ny 
and  with  I'tv  essor  Hastings  to  that  end.  With  their  well  known 
willingness.  I  may  even  say  anxiety,  to  ]>roduce  the  highc'-t  quality 
of  optical  work  and  to  make  any  im]irovements  lliat  may  be  suggested 


^^S 


..'.J .  -^.-   m.i'  '■■,'■ 


STAR  IM.iai-:  /.V  SPF.CTROCKM'HIC  WOKS 


■;»5 


to  tlum,  thi'  Hraslu'ur  Company  arc  undrrtakini,'  lo  niakc  a   in  w 
corrccting-lens  to  computations  hy   F'rofis>or  Ilasiinu's,  to  wlumi   I 
I  ;im  vcrv  much  indebted  for  criticisms  and  suu'i^i-iion^  on  tlu-  pn  sent 
paper.     1  may  say   that    I'rofessor   llastinus   finds  a    very   markid 
uKrcement  between  his  computed  data  of  tlie  objecti\f,  lolor  dirves. 
and  chromatic  dilTerences.  and  my  observations,     lie  explains  tlie 
failure  of  the  correctint;  lens  to  compensate  for  the  cliromatie  dilUr- 
ences  of  focus,  which  it  was  comi>uted  to  do,  by  tile  l.,ct  llial  lliis 
lens  has  to  correct  the  errors  of  an  objective  of  marly  tifty  linies  the 
area,  that  the  small  dej)artures  of  tlu-  wave  >urface>   from  ,i    true 
suhere  have  grown  enormously  when  tliese  surfaces  liaw  contracttd  to 
one-t'iftieth  their  original  area,  and  that  a  very  jxTfecl  cornel  ion  by 
spherical  surfaces  can  hardly  be  hoped  for.     lie  tliink.-,  iiowe\er, 
that  considerable  improvement  can  be  effecte<l,  and  1  have  no  (!(iul)l 
mvself  that  he  and  tiie  Brashear  Company  can  do  much  l)etter  ih.in 
he  says  when  they  have  (luantitative  valuis  of  the  existing  aberrations. 
The  reason  for  jiublishing  this  paper  in  its  present    incomplete 
form,  before  the  new  correcting-lens  is  ready,  is  to  bring  Infore  -hilar 
spectroscopists  the  important  matter  of  the  si/e  and  character  of  tlie 
star  image  given  by  their  telescojies.     I  have  gone  fully   into  tlie 
details  of  the  investigation  and  exi)lained  the  dil'liculties  that  ar()>e 
with  the  means  of  overcoming  them,  in  ordir  to  sniootii  the  way  for 
similar  investigations  into  the  character  of  the  star  image  given  by 
other   svstems   of   objective   and    correcting  Kns.     It   seems   to   me 
extremely  probable  that,  in  the  major  j)art  if  nut  all  of  the  telescoji.  s 
employed  in  spcctrographic  work,  aberrations  of  the  s  inu-  or  a  similar 
nature  are  present.     If  a   correcting  Kns  compute!   to  compensate 
for  the  chromatic  dilTerence  fails  in  one  case,  it  is  jio.-sible,  even 
imibable,  that  it  may  fail  in  others,     .\nother  basis  for  this  bdicf 
is  a  comparison  of  the  relative  e.\po>uri-  times  re(|uirecl  for  different 
installations  taking  into  account  si/.e  of  object  glass,  slil-widlh,  and 
disper>ic)n  of  the  spectrograph.     I  am  well  aware  that  such  a  ccmipari- 
son  mu>t  necessarily  be  incomiilete,  and  the  results  reached  subject 
to  an  uncertainty,  say,  of  23   per  cent.,  owing  to  the  dilfuulty  of 
comparing  dilTerent  installations  under  ditferent  conditions  of  seeing, 
etc.     We  have  already  seen  how  important  a  part  is  i)layed  by  atmos- 
pheric disturbances  in  enlarging  the  star  image  so  that  the  linear 


T-H 


2ib 


J.  S.   riASKETT 


(li  iim  lir  of  ilic  imi.m'  incruascs  marly  in  pre  ortion  with  tlu'  focal 
kiVi,'lli.  ami  llurcfon'  a|>|>r()\iiii  itily,  as  llu-  ratio  of  aiK-rturf  to  focal 
kni,'tli  (Iocs  not  vary  much  in  larj^'c  instrununts,  with  the  diameter 
of  tlu-  object  j,'l  ->.  {'onsc<|iiently,  the  !'ecl!\e  value  of  increase 
of  ai)erlurt'  is  not  proiiortion  tl  to  the  increase  of  area,  Init  mori'  nearly 
to  the  increase  of  (liaineter,  which  w.is  accord inj^l)-  used  in  the  compari- 
son. So  far  as  rei;  irds  the  relative  dispersion  of  different  instrununts, 
the  I  \po>ure  time  was  taken  as  directly  pro|)()rtional  to  the  linear 
di>per~ioii,  presuinini;  the  same  iieij^ht  of  s]>ectrum  in  each  case. 
No  accouni  w,is  taken  of  tlu-  difference  in  the  loss  due  to  al)sori)tion 
and  rellection  in  tlu-  prism  train,  allhouuli  this  may  he  quite  important 
in  >onie  cas(-s.  The  (-\iio-~ure  tinu-  reijuind  was  taken  as  inversely 
|)rop()nional  to  the  slit  width,  and  this,  as  one-  of  tlu-  e.\[K-rinu'nts 
iletailed  al>o\e  shows,  is  probably  nearly  in  accordance  with  the 
facts.  In  the  following  Table  \',  data  of  llu-  \arious  cc|uipments 
whivli  are  and  have  been  us(-d  -.  radial  \elociiy  work,  so  far  as  they 
were  awiilable  to  the  writer,  appear,  but  thise  <lala  an-  incompK-te 
and  may  in  some  cises  be  in  error,  although  probably  not  to  a  marked 
degrei-. 

I  AUI.K   V 

I'llMI'Md-^ON    (IF    I'.lFlrll  S(  II  S   ciK    I NST  \1  I  \Tli  >.>S 


1    'jUipltK  lU 


( )n.i\v.i 15 

N'rrkfs         40 

l.i(k    \U 

l,..u,ll        J  I 

NiA.iil .'5 

He  inn  1  -■ 

I'lilkiiw.! ;o 

I,..nl      I.'* 


Aitti.i!  Kxposurt' 
Required 


'"^&-     "Ir     all..Ms 

lit  tit  lilf 


1 

I 

1 S .  (1 

0 

.o.'q 

I 

i^oni 

^loni 

Om 

2. (.7 

-   I 

10  s 

o,!,S 

0.42 

7> 

■  I."; 

15 

-'     1 

5   7'' 

'  -•  5 

o-.i 

0.62 

-'  >  ■ 

-  ^  . 

4.-' 

L     <i 

-    i'' 

111 

'^-5 

1  ,C2 

1  JO 

I  20 

20  i 

I  -fi; 

-   T'** 

14  (1 

.0J5 

0    7(1 

70 

75 

I.S 

0  s 

0  (., 

1  >   - 

CJO 

1    1)1 

7> 

75 

15 

2  0 

-t-C 

I  i  0 

C20 

0  .Si) 

("{  ' 

".> 

!■; 

o.S,( 

0  (») 

iS  (1 

CJ^ 

1     20 

liO  ' 

60:- 

4 

Tlu-  above  comparison  shows  that  the  Lick,  Bonn,  and  Lord 
etiuipnu-nts  in  pracl'uc  ■.\y\ixum\\  more  iu-arl\-  the  llu-ort-tical  el'liciency 
than  the  Ottawa,  but  tlu-  Verkes,  Lowell,  \ewall,and  I'ulkowa  depart 
farther  from  it. 


HPPHVI 


STAR  ni.\GE  7.Y  SPECTROGRArUIC  WORK 


:^i: 


'I'hcrf  strms  tluri'fori'  rcasonahlc  j^rouiiu  for  l)i  lit\  itiLl  that  i  "ii- 
si(lrral)lc  imi)n)vi'nn'nt  in  tln'  tlVuicmv,  aiul  lonsidrraMi-  imrtasc  in 
ihe  r  mi^L'of  tlu'  majority  of  siKctro,i;ra|ilii(  ((iiiipnnnts  i.ji  hcatiaimd 
l)v  '  )okini^  into  llif  i  liarat  tcr  of  the  star  ini...i^c  j^ivcn  l>y  the  coniimsin^; 
ss>ti'm.  Although  llu'  lx  u  t  i-lTicl  of  atniosphiric  (li-turhann  ■-  on 
the  I'lTfctivr  dianu'tiT  of  thi'  star  inia^f  is  ditTKull  of  di  li  rminalion, 
I  fnl  satisfifl,  if  I  can  obtain  a  corrtc  tini:  lens  tliat  will  '^ivc  a  -t  ir 
ima^i'  roasonahly  free  from  alnrration,  that  the  (\|i<)>iirr  limr-  n- 
(luiri'd  hire  can  he  very  materially  ndiuid,  I  hojie  hy  50  )>«  r  unt., 
;ind  I  see  no  reason  wliy  a  similar  or  even  j^nati  r  imiirovemi  ni  could 
not  be  elTected  in  some  of  the  other  e(|uiiinnnts. 

1  acknowledfje  with  jiUasure  my  indel)tednes>  to  Or.  \\  .  I'.  Kinj;, 
the  Director  of  the  Observatory,  for  hel|)  and  encoiirai^munt  in  tin- 
prosecution  of  the  work,  and  to  Mr.  W.  K.  Harper  for  niakini,'  diijili- 
cate  measures  for  comparison  purjxjses  on  some  of  tiie  te^t  plates. 

Dominion  ()iiSKi;\  aiokv,  (>iia«a 
January,  1^07 


